When hydrocarbon stream passes through an acidic zeolite catalyst, two typical chemical reactions occur. First, the paraffinic and i-paraffinic portions of the stream crack down to form light olefins. Second, the naphthenic portion is dehydrogenated to form cyclic or chain olefins. Some of these olefins may combine and rearrange to form aromatics, such as toluene and xylenes. Such aromatic growth is different from the aromatic alkylation pathway in typical methanol-to-gasoline (MTG) where highly methyl-substituted benzenes are normally the preferred products. It is known that toluene and xylenes are desirable for high octane, while durene (1,2,4,5-tetramethyl benzene) is not. The generation of additional desirable species, such as toluene and xylenes, is always a positive direction for octane enrichment. Besides, naphthenic components are known to be undesirable for high octane rating. In addition, iso-durenes are known to be bad actors for viscometric properties, as they increase the fuel viscosity. The conversion of undesirable naphthenes into desirable species, such as toluene and xylenes, is one of the most effective way to enhance octane and improve viscometric properties of fuel made from synthesis gas (synfuel). Besides aromatization, the acid zeolite catalyst has the capability for transalkylation, where methyl groups may be interchanged among aromatic moieties intra- or inter-molecularly. The zeolite-forming pathway is summarized in FIG. 1.
Zeolite-forming has been extensively studied by Zeosit and exercised in many refineries, located mostly in Eastern Europe and Russia. The economic evaluation has also been conducted in a comparison to other reforming technologies based on precious metals (Stepanov et al., Chemistry for Sustainable Development 13:505-518 (2005); Rovenskaja et al., Chem. Ind. 57(9):399-403 (2003); Erofeev et al., XVIII International Scientific Symposium in Honor of Academician M.A.Usov PGON2014). In contrast to Pt-reforming, zeolite-forming does not require the expensive catalysts and the hydrofining stages to remove sulfur and nitrogen species from the raw materials. The regeneration of Pt-based catalyst is difficult and not cost effective. Due to the light gas loss, zeolite-forming needs to be optimized between the recovery and efficiency in octane boosting. In other words, the use of zeolite-forming alone may not be a sufficient way to improve octane rating in synfuel.
Therefore there is a need to increase octane rating in synfuel without the drawbacks discussed above.